The use of contactors with relays in industrial control applications is very popular. The proper choice of connection mode will enhance system stability and increase lifespan. Contactors are commonly used for switching of large loads such as motors and heaters or lighting, whereas relays usually perform the signal conversion and control functions.

It is reported that according to the International Electrotechnical Commission goodwill, over eighty percent of industrial electrical control circuits utilize contactors and relays with some degree of co-operation. The variables which influence the final connection will include the voltages of the different devices, contact load ratings, and protection mechanisms of circuits. Therefore, it is of utmost importance to ensure voltage matching, select contacts rationally, and carry out a stable connection; thus, the long-term trustworthy operation of the concerned system can be assured.

Voltage Matching

The appropriate voltage specification must be confirmed first in any contactor and relay connection. Any mismatch in voltage will, at best, keep the relay or contactor from pulling in; at worst, it may damage the equipment, or worse, endanger personnel with an electrical accident. The coil voltage under relays is usually classified as 12V DC, 24V DC, 110V AC, and 220V AC, with certain special selection criteria for different application cases. Certainly, in the PLC (Programmable Logic Controller) control systems, over 75% of the used relays are 24V DC. Their merits include lower power consumption, super anti-interference ability, and almost all definitions in industrial engineering standards. In control systems that apply high voltages, like elevator control cabinets or larger machine tools, 110V AC and 220V AC relays are more frequent.

Contactor coil voltage must also match the voltage from the relay. In other words, if the relay contact is rated for 220V AC and the contactor operating voltage is 24V DC, either a transformer or a solid-state relay (SSR) must be utilized for conversion. To explain this further, the ABB A9 series contactor coil power consumption is considered around 3~5W. If there is a non-sufficient relay contact rated current, it burns out the switches. It was shared by Siemens in Germany that during the starting an inrush current for a contactor coil might go up to 7-10 times its rated current. Therefore, there must be a safety margin kept while designing the power supply to avoid voltage drops and to avoid contact welding problems.

Select The Appropriate Contacts

In relay circuits, contacts are normally classified as normally open (NO) and normally closed (NC). Each type of contact serves different applications. In most control circuits, for example, motor start and lighting control circuits, normally open contacts are preferred, which means that they close when the relay is energized and open otherwise. In contrast, normally closed contacts work inversely; they close when the relay is not energized and open when it is energized, making them suitable for emergency stop or power-off protection applications. A very good example would be where a 7.5kW three-phase motor control realizes proper operation based on a relay with the right contacts; otherwise, the wrong choice may hinder motor startup or shutdown.

The current ratings of contacts are another serious issue for relay contacts. If a relay contact is rated for 5A and the contactor coil has an operating current of 7A, the contact will soon get damage. According to the National Electrical Manufacturers Association (NEMA) standards, the rated current of relay contacts should be 1.2 times the load current at least to assure long-term reliable operation of the contacts. In some high-frequency switching applications, say, production line control systems, double contact relays or solid-state relays are being used to minimize the wear on contacts and make the overall system more reliable.

Connect the Relay Control Circuit

The relay control circuit includes coil power, contact output, and signal input. With proper wiring, electromagnetic interference will be reduced and signals will be stabilized. The Siemens 3RH series relays, for example, have a coil resistance usually from 1.2k to 1.8k. When energized, the current through the coil is approximately 20mA, at a voltage of 24V DC, while a relay coil at 220V has an approximate coil current of 10mA. In employing the relay control circuit, however, it should be made sure that the value of the driving signal employed gives enough current to energize the relay; otherwise, working stably, the relay would brake, therefore hampering system operation.

When using the relay for DC control, switching off the coil may therefore generate back high voltage that may damage PLCs or the control circuit. Experimental evidence obtained from industrial control system fault analysis indicates that the induced voltage generated at the instant of disconnection for a 24V DC relay coil can attain values of 300V. Therefore, 1N4007 diodes connected in parallel across the DC relay coil are recommended for protective elimination of back EMF. An RC snubber network is suggested for AC relays (e.g., Schneider RC220V) to diminish switching arcing and improve relay lifetime.

Connect the Contactor Main Circuit

The main circuit of a contactor helps control the load current, thus the way it is wired affects the safety and stability of the system. For example, taking the Schneider LC1-D09 contactor as an example, it has a rated current of 9A, and the main contacts can withstand up to a maximum voltage of 690V AC, which can be used for motors, lighting, or heating equipment.

For a 7.5kW three-phase asynchronous motor, the minimum rated current of the main contacts on the contactor would have to be 15A. (Calculated from the equations I = P / (√3 × U × cosφ), assuming cosφ = 0.85). Therefore, if we select a contactor with a 10A rating, excess load current, contact overheating, and even welding may occur. According to IEC standards, long exposure to temperatures above 65° would adversely affect contactor life. Therefore, the installation of the contactor should be checked for its operational status with an infrared thermometer, and, finally, appropriate wiring terminals should be selected in order to minimize resistance in the contacts.

Add Self-locking and Protection

These facilities are essential for improving reliability in the control system. The self-locking circuit is a well-established common circuit for motor start control. For instance, in the control circuit of a 5.5kW water pump, the self-locking circuit is formed by connecting the NO auxiliary contact to the same parallel branch as the start push button. Once the start button is pressed, the contactor remains energized even after the start button is released until the stop button is pressed.

Overload protection is also required. For example, the thermal relay LRD12 should be used so that it can interrupt the motor when overload operating conditions arise. Research on motor protective maintenance states that in an industrial application, 60% of motor outages are due to overload or phase loss of supply. For improving the system's interference ability, it can be considered to connect an RC Suppressor in parallel across the contactor coil. Experimental results show that the RC-uppressor reduces the voltage surge peak by more than 50%, thus reducing EMI.

Testing and Debugging

Testing and debugging are paramount to ensure correct wiring. In-power tests could use a multimeter across the relay coil to measure voltage into the rated range. For example, a 24V DC relay is energized somewhere between 22.8V and 25.2V; a voltage falling out of this range would mean that either the relay may fail to pull-in (if voltage is below) or may burn-off (if voltage is above). The pull-in time of the relay can be considered in function testing, which should be, in general, not longer than 30ms; if this time exceeds 50ms, it is possible that something is wrong with input signal delays.

Besides, it is critical to monitor the contacts' temperature. If the contact temperature exceeds 80°C for extended periods, increased contact resistance would accelerate contact ageing. So it will also be important in practice to use an infrared thermometer to periodically test the contact temperature to ensure that it is in the safe range. Any unusual heating recorded would then warrant checking whether the wiring terminals are tight, or if the contacts are oxidized, or replacing the relay with one of higher capacity to reduce load pressure.